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Cite this: Chem. Commun., 2015,
51, 1830
Received 18th November 2014,
Accepted 11th December 2014
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Switching adhesion and friction by light using
photosensitive guest–host interactions
Johanna Blass,ab Bianca L. Bozna,a Marcel Albrecht,c Jennifer A. Krings,d
Bart Jan Ravoo,d Gerhard Wenzc and Roland Bennewitz*ab
DOI: 10.1039/c4cc09204j
www.rsc.org/chemcomm
Friction and adhesion between two b-cyclodextrin functionalized
surfaces can be switched reversibly by external light stimuli. The
interaction between the cyclodextrin molecules attached to the
tip of an atomic force microscope and a silicon wafer surface is
mediated by complexation of ditopic azobenzene guest molecules.
At the single molecule level, the rupture force of an individual
complex is 61 10 pN.
Surfaces with switchable adhesion or friction characteristics
present a new field of research and promise great potential in
the field of adhesives or tribological systems controlled by
external stimuli. Self-assembled monolayers are one promising
approach to provide surfaces with switchable functions, examples
being the switching of wettability1 or of nanomechanical
properties.2 In this field, azobenzene-derived systems are of
special interest as azobenzene has been shown to change its
configuration instantly but reversibly by irradiation with ultraviolet (UV) and visible (VIS) light, respectively,3 without generating
any side products.4 Thus, azobenzenes have been used in supramolecular assemblies,5 in covalent functionalization of metal
surfaces,4 or for the switching of bioactivity by controlling the
binding of azobenzene modified glycoconjugates to cyclodextrin
monolayers.6 Atomic force microscopy (AFM) allows to study
mechanical properties of surface-linked azobenzene-derivatives
at the single molecule level.7
In this letter, we present a novel surface functionalization
based on cyclodextrin assemblies with photo switchable friction
and adhesion characteristics mediated by ditopic azobenzene
connector molecules in solution. Azobenzene moieties can be
isomerized reversibly from trans to cis states by irradiation with
light at 365 nm and from cis to trans at 455 nm. The rod like trans
state of the azobenzene is apolar and binds efficiently into the
cyclodextrin cavities whereas the bent cis form is much more
polar and sterically hindered from complexation.8 Force
measurements by AFM are sensitive enough to detect single
host–guest interactions. These interactions have been studied
in detail for various guests in cyclodextrin (CD) hosts.9
We use AFM to study the molecular mechanisms underlying
friction and adhesion. A b-cyclodextrin (b-CD) monolayer is
attached to both the oxidized silicon tip of the AFM and the
oxidized silicon wafer substrate, see the scheme in Fig. 1(a). The
attachment is realized by first tethering a silane adlayer with
isothiocyanate groups in tetrahydrofuran to the silicon oxide
surfaces. The b-CD molecules carry an amine functionality10
which is able to form a stable thiourea bond by reaction in
a
INM - Leibniz-Institute for New Materials, Saarland University, Campus D2 2,
66123 Saarbrücken, Germany. E-mail: [email protected]
b
Physics Department, Saarland University, Saarland University, Campus D2 2,
66123 Saarbrücken, Germany
c
Organic Macromolecular Chemistry, Saarland University, Campus C4 2,
66123 Saarbrücken, Germany
d
Organic Chemistry Institute, University of Münster, Corrensstrasse 10,
48149 Münster, Germany
1830 | Chem. Commun., 2015, 51, 1830--1833
Fig. 1 (a) Schematic representation of the experiment. Tip and surface are
functionalized by b-cyclodextrin molecules. Both end groups of ditopic
guest connector molecules form complexes with b-cyclodextrin molecules at tip and surface, resulting in adhesion and friction. (b) Molecular
structures of the compounds.
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water with the isothiocyanate groups at the surfaces. Attractive
interactions between the b-CD molecules at tip and surface arise
from complexation of ditopic guest connector molecules, see
Fig. 1(b). The synthesis of the ditopic guest molecules and their
photo-responsive complexation with cyclodextrin molecules
has been described elsewhere.8 Typical association constants
for trans-azobenzene derivatives with b-CD are of the order of
2.5 103 M 1,11 while the binding affinity of cis-azobenzene is too
small for quantification. For the binding of azobenzene in b-CD
monolayers an association constant of 5.8 103 M 1 has been
reported.6 The ditopic connector molecules carry two hydrophobic
end groups which form a specific non-covalent bond between two
opposing b-CD cavities via hydrophobic interactions.
All force microscopy experiments were performed at room
temperature, using a JPK NanoWizard3 AFM and a rectangular
silicon cantilever with a nominal force constant of 0.2 N m 1.
Normal force measurements were calibrated following the methods
introduced by Sader and co-workers.12 Lateral force measurements
were calibrated using the wedge method in air13 (sample TGG01
from Micromash, Sofia, Bulgaria) and applying the correction
factor for experiments in liquid introduced by Tocha et al.14
A typical example for a force–distance curve is shown in
Fig. 2(a). The force between tip and surface is recorded while the
AFM cantilever is approached to and retracted from the surface.
Two stepwise increases of the attractive force are observed when
the tip apex approaches the surface, indicating a molecular
snap-into-contact through complexation events. The rupture of
Fig. 2 (a) Representative force–distance curve recorded in the guest
connector solution. Formation of non-covalent bonds can be observed
upon approach, a high pull-off force and the subsequent rupture of
multiple complexes are observed during retraction. (b) Detail of the retract
curve in (a), showing the last rupture event and the related force drop
Frupture. (c) Histogram showing the distribution of Frupture. The solid line
represents the sum of Gaussian curves fitted to the peak.
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b-CD–azobenzene complexes occurs in multiple steps during
the retraction of the AFM cantilever from the surface. Each
force drop reflects the rupture of multiple or single complexes.
To determine the characteristic rupture force for an individual
complex, the distribution of the last force drops, labelled Frupture
in Fig. 2(b), is plotted as a histogram in Fig. 2(c). Only the last
rupture event of each curve is considered to ensure that the
cantilever is fully relaxed after the rupture event. The histogram
exhibits multiple maxima with equidistant spacing corresponding
to the rupture of a single complex, parallel rupture of two complexes etc. The maxima lie at multiples of 61 10 pN, which is the
characteristic rupture force for a single complex. In an ongoing
study of ditopic guest connector molecules with adamantane
end groups we have found a single complex rupture force of
82 10 pN. The lower rupture force for the azobenzene
connector is consistent with the lower binding constant for
complexes of azo derivatives with b-CD of K = 2.5 103 M 1
(ref. 11) as compared to the one for adamantane derivatives
with b-CD of K = 4.6 104 M 1.15
Previous force microscopy studies with various monotopic
guests have reported rupture forces ranging from 39 to 102 pN,
where adamantane guests form the strongest complexes with
cyclodextrin molecules.9a The maximum of the histogram
indicates the most probable force to rupture one of two complexes connected in series, while the most probable force to
rupture one single azobenzene–b-CD complex would probably
be a little higher.
The influence of the connector molecules and of their lightinduced isomerization on the maximum pull-off force (1.8 nN in
the example in Fig. 2(a)) is summarized in Fig. 3. Control experiments were performed in ultrapure water. Connector molecules
were diluted in dimethyl sulfoxide (DMSO) and added to the
ultrapure water in a ratio of 1 : 10, resulting in 10 mM concentration.
Fig. 3 Histograms of pull-off forces recorded in a series of experiments for
a b-CD functionalized AFM-tip and surface in a control experiment in pure
water, in a solution of azobenzene connector molecules, after irradiation
with UV light (367 nm), and after irradiation with VIS (454 nm) light.
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Each histogram reflects the distribution of pull-forces from
1200 force–distance curves (pulling rate 1 mm s 1, maximum load
0.5 nN) recorded in 70 minutes. The histograms show the significant effect of the ditopic connector molecules on the adhesion force.
In the control experiment, non-specific tip–sample interactions
result in a narrow distribution of pull-off forces around 0.35 nN.
After adding the guest molecules the spectrum of adhesion forces
becomes much wider with pull-off forces between 0.5 and 1.5 nN,
some even up to 2.5 nN. Irradiation with the light of a UV LED
(peak intensity at 367 nm, 10.5 nm width) results in a decrease of
pull-off forces to the values of control experiment. By subsequent
irradiation with VIS light (broad-spectrum white light LED with
peak intensities at 454 nm and 553 nm) the system was switched
back to the high adhesion state with pull-off forces ranging again
from 0.3 to 2.5 nN. While the low pull-off force values around
0.3 nN observed in the control experiment are not observed after
initially adding the connector molecules, they are observed after
the sequence of UV and VIS irradiation. The transition of azobenzene end groups from the cis to the trans state under VIS
irradiation may not be complete and, therefore, non-specific
interactions may arise from competition of ditopic complex
formation with only partially isomerized connector molecules.
Only a low concentration of guest molecules in the water–
DMSO solution (10 mM) is required to achieve these significant
effects on adhesion. About 60 complexes contribute to adhesion.
This rough upper bound estimate can be made by taking the
product of the maximum packing density of b-CD molecules of
0.33 nm 2 and an estimated contact area derived from the tip
radius of 200 nm2 and by assuming that most b-CD molecules are
complexed. While changing the light stimuli, the AFM-tip was
retracted about 3 mm from the surface, resulting in a time gap of
less than five minutes between each series of force–distance
curves. The results presented here were achieved with illumination of tip and sample from the side. Quantitatively similar
results were obtained for a borosilicate glass substrate functionalized in the same way, where tip and sample were illuminated
through the substrate.
The molecular complex formation and its switching by light
stimuli also lead to a significant friction contrast. The result of
a series of friction force microscopy experiments is summarized
in Fig. 4. The friction force is determined from the torsional
bending of the AFM cantilever when the tip slides in contact with
the surface.16 In this configuration, friction can be switched
reversibly by a factor of more than two by irradiation with UV
and VIS light. The friction force decreases upon UV irradiation
almost to the value of the control experiment without azobenzene
connector molecules. The measurements were performed by
scanning lines of 50 nm back and forth, i.e. several times the
estimated contact diameter, in 1 second. The normal load was
kept at a low value of 0.5 nN in order to minimize non-specific
and deformation contributions to the friction signal. The AFM tip
was retracted for 1 minute while switching the irradiation
wavelength. Typical lateral force traces are shown in Fig. 4(b)
for a control in pure water and for irradiation with UV and VIS
light in the guest molecule solution. The complex formation by
the guest connector molecules under VIS irradiation does not
1832 | Chem. Commun., 2015, 51, 1830--1833
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Fig. 4 (a) Light-responsive friction between b-CD functionalized AFM-tip
and silicon surface in azobenzene connector molecule solution. The
friction force is calculated as the average lateral force during one forward
and one backward scan over 50 nm. The error bars represent the standard
deviation of 512 scan lines. (b) Typical lateral force traces for forward and
backward scan in pure water as control experiment (black line) and in
azobenzene connector molecule solution during irradiation with VIS light
(red) and UV light (blue).
only cause higher average friction, but also changes the characteristic appearance of the lateral force traces.
While the non-specific interactions in the control experiments
lead to a smooth lateral force curve with some minor fluctuations, the rupture and rebinding of guest–host complexes produces irregular stick-slip motion of the AFM tip which is revealed
as saw-tooth characteristic of the lateral force. Typical force drops
in the stick-slip force trace have values scattering around 250 pN,
comparable to the force drops observed in the force–distance
curves in Fig. 2(a) when multiple non-covalent bonds were broken.
The friction process can thus be described in the following way. The
AFM tip is stuck in place by multiple non-covalent bonds, while the
lateral force is built up by the pulling of the cantilever. When
the lateral force reaches a threshold value, one or usually multiple
bonds are broken and the tip slips to a new position where a
different configuration of non-covalent bonds leads to a further
sticking phase. This process is continuously repeated, as each new
position of the tip leads to rebinding with new b-CD molecules at
the surface. The force trace recorded in guest molecule solution
under UV irradiation reveals significantly lower average friction than
that recorded under VIS irradiation, but exhibits some signatures of
the stick-slip characteristic, reflecting that even under UV irradiation
a fraction of about 20% of the connector molecules is still in the
trans-state and contribute to the molecular binding process.17
In conclusion, we present a novel surface functionalization
of host monolayers with switchable friction and adhesion
properties mediated by photoresponsive connector molecules.
At the molecular level, the rupture force of an individual host–
guest complex formed by b-cyclodextrin molecules and the two
azobenzene end groups of the connector molecule is 61 10 pN.
The pull-off force between functionalized AFM tip and silicon
surface can be switched by a factor of five by irradiation with UV
and VIS light, similarly the friction can be switched by a factor
of two. The novel surface functionalization is a first example to
demonstrate active control of adhesion and friction by means
of macromolecular guest–host chemistry. The molecular design
of our approach allows for different guest connector molecules
binding the same surface functionalization, offering a great
variability in the molecular control of friction and adhesion.
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The authors thank the Volkswagen Foundation for financial
support through the program Integration of Molecular Components in Functional Macroscopic Systems. Furthermore, Devid
Hero is gratefully acknowledged for his synthetic support in the
preparation of some cyclodextrin derivatives.
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